Ok solved it myself... I swear I tried it before but it didnt work, maybe I made a typo somewhere earlier. This is the code to send a fixed attachment (in my example its mytext.txt) from your own server, the file needs to be in the same folder as the script in my example but you could add a path to it too.
The $gegevens is the string that will be the content of the mytext.txt file.
<?php
// Read POST request params into global vars
$to = $_POST['to'];
$from = $_POST['from'];
$subject = $_POST['subject'];
$message = $_POST['message'];
// Obtain file upload vars
$fileatt = "mytext.txt";
$fileatt_type = "text/plain";
$fileatt_name = "mytext.txt";
$headers = "From: $from";
// Generate a boundary string
$semi_rand = md5(time());
$mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
// Add the headers for a file attachment
$headers .= "\nMIME-Version: 1.0\n" .
"Content-Type: multipart/mixed;\n" .
" boundary=\"{$mime_boundary}\"";
// Add a multipart boundary above the plain message
$message = "This is a multi-part message in MIME format.\n\n" .
"--{$mime_boundary}\n" .
"Content-Type: text/plain; charset=\"iso-8859-1\"\n" .
"Content-Transfer-Encoding: 7bit\n\n" .
$message . "\n\n";
// Base64 encode the file data
$gegevens = chunk_split(base64_encode($gegevens));
// Add file attachment to the message
$message .= "--{$mime_boundary}\n" .
"Content-Type: {$fileatt_type};\n" .
" name=\"{$fileatt_name}\"\n" .
"Content-Disposition: attachment;\n" .
" filename=\"{$fileatt_name}\"\n" .
"Content-Transfer-Encoding: base64\n\n" .
$gegevens . "\n\n" .
"--{$mime_boundary}--\n";
mail($to, $subject, $message, $headers)
?>